The present invention refers to adjustment of the needle guard plate on a vertical axis hook or crochet of a lockstitch sewing machine.
In lockstitch sewing machines the crochet or hook has a peripheral xe2x80x9chook pointxe2x80x9d extending for a distance along the circumference of the hook. The hook, by rotating around its own axis, grips with said point the loop formed by the upper thread or needle thread, a loop which forms at the start of the movement of withdrawal or raising of the needle. The point of the hook must brush the needle to grip the loop of thread. Any bending of the needle, due for example to the force exerted thereon by a sideways translation of the fabric, could bring it to be situated on the trajectory of the hook point and could cause a collision with probable breakage of the needle or splintering of the hook point. In order to avoid this, a needle guard plate is provided which serves to prevent the needle from bending in the direction of the hook tip. The plate has an arched shape, in its working portion seen in a plan view, and rotates integrally with the hook body.
FIG. 28 is a top perspective view illustrating a vertical axis hook 1, according to the prior art, provided with a hook point 2, and a needle-protecting plate, or needle-guard plate, denoted by reference numeral 3.
Adjustment of the position of the plate in a radial direction with respect to the axis of the hook is generally required or desirable to adapt the hook to the type and size of needle used. The state of the art comprises various solutions for mounting plates on the hook body, so as to be able to perform said adjustment. Needle guard plates are generally considered to be grouped into three families according to the type of construction: a first family comprises so-called xe2x80x9cflange platesxe2x80x9d, formed by a sheet of metal bent into an L-shape which is placed with the flat part between the hook and its axial resting point on the sewing machine; a second family comprises xe2x80x9cflange-mounted platesxe2x80x9d wherein a flange is fastened to the rear part of the hook; a third family comprises the xe2x80x9cperipheral platesxe2x80x9d, obtained from sheet metal bent in an arch and fastened to the side of the hook body.
The subject matter of the present invention concerns the field of peripheral plates.
Peripheral plates are positioned on and fastened to the body of the hook. Some currently known and used methods of fastening will be described briefly.
According to a first method, the plate is mounted on the side of the hook by means of two head countersunk screws which centre the respective seats on the plate and therefore perform both the positioning function and that of fastening of the plate. The two screws are in proximity to the opposite end of the plate with respect to the hook point and leave the part of the plate near the hook point projecting. A variant of said system consists in fastening with four screws, two for each end of the plate.
In a second manner, the plate is positioned in a circumferential groove on the side of the hook and is fastened by means of two flat-headed screws, which therefore serve no positioning function, but only that of fastening.
As far as adjustment of peripheral plates is concerned, the following three systems are known and used.
a) The mounted plate is adjusted only by manual deformation of the bending curve of the projecting part with respect to the positioning and fastening system. Adjustment can therefore be only rough.
b) A third screw is placed on the side of the body of the hook at the opposite end of the needle guard plate with respect to the positioning and fastening portion. The screw, when it is loosened, presses on the tip of the plate and obliges outward deformation thereof. The screw normally requires a braking system (typically a locking thread) and holds the disadvantage of not being able to be applied to all hooks, in that it requires an adequate space, on the side of the hook and near the hook tip, a space which often is not available. Such an adjustment system is described in patents DE G 8116870 and IT 1151267.
c) In the case of the hook with a plate positioned in the groove, whilst positioning of the plate is performed by the groove and fastening of the plate by the flat-headed screw nearest the end of the plate, for fine adjustment use is made of the second screw, positioned between the fastening screw and the tip of the plate at the hook point. In this case the plate has a double bend which, without the second screw, would position it far to the outside of the hook tip. By screwing the second screw, on the other hand, the plate is forced inwards.
An object of the present invention is to reduce production costs of hook with an adjustable plate.
A further object is to be able to apply a finely adjustable plate on a hook where this was not hitherto possible because of problems of space.
The object has been achieved with a hook as stated in claim 1. Further new and useful characteristics are disclosed in the dependent claims.
In other words, in the hook of the invention fine adjustment of the peripheral needle guard plate is achieved without the need for a third screw in proximity to the point of the hook and with the sole function of adjustment, and without the need for a slot for positioning of the plate.
In one embodiment, three screws (two countersunk head for fastening and positioning and one for adjustment), all positioned on the portion of plate opposite that facing towards the hook point. The plate has a double bend and the third screw forces the plate inward.
In another embodiment, two countersunk head screws are used, one of which serves for positioning and fastening, whereas the second serves for positioning and adjustment. The plate has a double bend and the second screw forces the plate inwards.
In a third embodiment, two screws are used, one countersunk head for fastening and the second flat-headed for positioning and adjustment, having a cylindrical shape between the head and the thread which couples with a horizontal slot on the plate.
In a fourth embodiment two screws are used, the first being countersunk head and having a positioning and fastening function, whereas the second, serving for positioning and adjustment, works in thrust and is shaped with a cylindrical pin protruding from the head of the screw. Said pin enters a hole or slot in the plate and it is also possible from the pin to act to tighten or loosen said screw. The plate is naturally bent inwards and is forced outwards by loosening the screw.
In a fifth embodiment two screws are used, the first of which is countersunk head and serves for positioning and adjustment, whereas the second serves for positioning and adjustment. The second screw serves for xe2x80x9cdual effectxe2x80x9d adjustment: that is, it works both in traction and in thrust, depending on whether it is more or less tightened. The plate is bent to be level with the hook point with the adjustment screw half tightened. The adjustment screw has two flanges that remain one on the inside and one on the outside of the needle guard plate. The outer flange is flat in shape.
A sixth embodiment is similar to the fifth but the outer flange of the adjustment screw is conical.
The invention achieves the aforementioned objects, in particular it achieves lower production costs for the hook and allows simplified adjustment where it was not otherwise possible.